The technical field of this invention is magnetometry and, in particular, the nondestructive electromagnetic interrogation of materials of interest to deduce their physical properties and to measure kinematic properties such as proximity. The disclosed invention applies to both conducting and magnetic media.
Conventional application of magnetometers, specifically eddy current sensors, involves the excitation of a conducting winding, the primary, with an electric current source of prescribed temporal frequency. This produces a time-varying magnetic field at the same frequency. The primary winding is located in close proximity to the material under test (MUT), but not in direct contact with the MUT. This type of nondestructive electro-magnetic interrogation is sometimes called near field measurement. The excitation fields and the relevant spatial and temporal variations of those fields are quasistatic. The magnitude and phase (or the real and imaginary parts) of the impedance measured at the terminals of the primary winding (i.e., the measured voltage at the primary winding terminals divided by the imposed current) or the transimpedance (i.e., the voltage measured at a secondary winding terminals divided by the imposed current in the primary winding) is used to estimate the MUT properties of interest.
The time-varying magnetic field produced by the primary winding induces currents in the MUT that produce their own magnetic fields. These induced fields have a magnetic flux in the opposite direction to the fields produced by the primary. The net result is that conducting MUTs will tend to exclude the magnetic flux produced by the primary windings. The measured impedance and transimpedance at the terminals of the sensor windings is affected by the following: the proximity to the MUT, the physical properties (e.g., permeability and conductivity) of the MUT and the spatial distribution of those properties; the geometric construct of the MUT; other kinematic properties (e.g., velocity) of the MUT; and the existence of defects (e.g., cracks, corrosion, impurities).
The distribution of the currents induced within conducting MUTs and the associated distribution of the magnetic fields in the MUT, in the vicinity of the MUT, and within the conducting primary and secondary windings are governed by the basic laws of physics. Specifically, Ampere""s and Faraday""s laws combined with Ohm""s law and the relevant boundary and continuity conditions result in a mathematical representation of magnetic diffusion in conducting media and the Laplacian decay of magnetic fields. Magnetic diffusion is a phenomena that relates the distribution of induced currents in conducting materials to the distribution of the imposed and induced magnetic fields. Laplacian decay describes the manner in which a magnetic field decays along a path directed away from the original field source.
Magnetometers, such as eddy current sensors, exploit the sensitivity of the impedance or trans-impedance (measured at the sensor winding terminals) to the physical and geometric properties of the MUT. This is sometimes accomplished by using multiple temporal excitation frequencies. As the primary winding excitation frequency is increased, the currents in a conducting MUT exclude more and more flux until all the induced currents in the MUT are confined to a thin layer near the surface of the MUT. At frequencies for which the induced currents are all at the surface of the MUT, the MUT can be represented theoretically as a perfect conductor. In other words, at high enough frequency variations, the conductivity of the MUT will no longer affect the impedance or transimpedance measured at the sensor windings.
This effect has been used in proximity measurement relative to a conducting media. Measurement of proximity to a metal surface is possible at a single excitation frequency, if that frequency is high enough that the MUT can be treated as a perfect conductor. For proximity measurement at lower frequencies, it is necessary to account for the effects of the conductivity of the MUT on the measured impedance, either by physical modeling or by calibration.
In applications requiring the measurement of conductivity, it is necessary to operate at frequencies low enough that the measurements at the terminals of the conducting windings are sensitive to the MUT conductivity. Such applications include the monitoring of aging in conducting media, as well as the direct measurement of conductivity for quality monitoring in metal processing and manufacturing process control. For example, the accurate measurement of the case depth (e.g., the thickness of a heat-affected zone at the surface of a metal after heat treatment) requires a sensor winding geometry and excitation conditions (e.g., frequency, proximity to the MUT) that produce the required sensitivity to the conductivity and thickness of the heat-affected zone.
Two methods are available for determining the desired conditions: (1) experimentation and calibration, and (2) physical modeling and response prediction from basic principals. In practice, each of these techniques has met with some success. The principal limitations of experimentation and calibration are the need for fabrication of expensive calibration test pieces (standards) for each new application, the relatively small dynamic range (i.e., the small range of permissible MUT property variations over which the measurement specifications can be met), and the inaccuracies produced by variation in uncontrolled conditions such as temperature and lift-off errors.
The principal limitations of the physical modeling approach are the inaccuracies introduced by modeling approximations and the existence of unmodeled effects. These limitations are most severe for sensor winding constructs that are not specifically designed to minimize unmodeled effects.
In spite of these limitations, the successful use of conducting windings driven by a current source, as in eddy current sensors, to measure physical and kinematic properties has been widely demonstrated.
For example, eddy current sensors have been used to measure the thickness of conducting strips of known conductivity, as disclosed in Soviet Patents 578,609 and 502,205. Eddy current sensors have also been used for flaw detection, as disclosed in U.S. Pat. No. 3,939,404. Other eddy current sensor applications include measurement of the conductivity-thickness product for thin conducting layers, measurement of the conductivity of conducting plates using calibration standards, and measurement of proximity to conducting layers. Such sensors are also used in proximity measurement for control of machines and devices.
The ability to resolve distributions of parameters and properties of different layers in multi-layered materials has been addressed in U.S. Pat. No. 5,015,951. The referenced patent introduced the concept of multiple wavenumber magnetic interrogations of the material of interest, by imposing several different spatial magnetic field excitations, using multiple preselected sensor winding constructs, each with a different wavelength.
There is a substantial need for enhancements to the measurement performance capabilities of magnetometers. This includes the ability to measure (1) the conductivity and thickness of thin metallic layers independently to improve quality control in deposition and heat treatment processes (in practice, only the product of conductivity and thickness can be measured for thin layers for which the conductivity-thickness product is below a certain threshold); (2) more than one property independently with reliable and predictable performance over a wide dynamic range to provide a more accurate characterization of the MUT; (3) geometric or physical properties over a wide dynamic range without calibration to reduce cost and measurement setup time; (4) material properties such as permeability and conductivity of ferrous metallic layers or conductivity of deposited metallic layers, for quality control and property monitoring after processing (e.g., in situ monitoring of permeability for sheets of transformer core alloy, or conductivity measurement for thin metallic layers of different conductivity that form on metallic surfaces during heat treatment); (5) the thickness of conducting layers or heat affected zones on conducting substrates that do not have conductivities which are significantly different from that of the surface layer, to control heat treatment and monitor MUT properties; (6) the independent measurement of both the conductivity and height (i.e., the distance between the sensor windings and the MUT) of a conducting layer, to accurately account for lift-off affects in applications such as crack detecting (i.e., air gaps between the sensor windings and the MUT surface); and (7) measurement of kinematic properties such as proximity and relative velocity to conducting and magnetic media for actuator and process control.
Furthermore, there is a need for measurement methods that provide estimates of the actual physical properties of the MUT. Current techniques often measure xe2x80x9ceffectivexe2x80x9d properties that are only indirectly related to the actual physical properties (e.g., permeability and conductivity at a specified excitation frequency). These xe2x80x9ceffective property measurements often provide insufficient characterization of the MUT. For example, multiple temporal excitation frequencies are often used to obtain estimates of conductivity or permeability. This is not acceptable if these physical properties vary with temporal excitation frequency. In applications such as monitoring of aging and fatigue in ferrous and nonferrous metal alloys, it may be necessary to completely characterize the dispersive properties of the MUT, including the variations of conductivity and permeability with temporal excitation frequency. Thus, a technique is required that provides accurate estimates of actual physical and geometric properties of the MUT from measurements at a single temporal excitation frequency.
To overcome the aforementioned limitations in current practice, magnetometers must provide increased sensitivity, selectivity and dynamic range as well as the capability to measure actual MUT properties without calibration when required. Note that sensitivity is defined herein as the incremental change in the transimpedance measured at the sensor terminals in response to an incremental change in the geometric or physical MUT property of interest. Selectivity is defined herein as a measure of the ability to independently estimate two distinct properties (e.g. conductivity and thickness of a thin conducting layer). Dynamic range is defined herein as the range of MUT properties over which sufficient sensitivity and selectivity can be achieved.
In accordance with the present invention, apparatus and methods are disclosed which provide increased sensitivity, selectivity and dynamic range for non-contact measurement of actual physical and/or kinematic properties of conducting and magnetic materials. The disclosed invention is based upon various methods for increasing sensitivity, selectivity and dynamic range through proper construction of the magnetometer sensor and proper selection of operating point parameters for the application under consideration.
In one embodiment, a measurement apparatus for measuring one or more MUT properties includes an electromagnetic winding structure which, when driven by an electric signal, imposes a magnetic field in the MUT and senses an electromagnetic response. An analyzer is provided for applying the electric signal to the winding structure. A property estimator is coupled to the winding structure and translates sensed electromagnetic responses into estimates of one or more preselected properties of the material. In accordance with the present invention, the temporal excitation frequency of the electric signal applied to the winding structure is proximal to a transverse diffusion effect (TDE) characteristic frequency of the winding structure.
The TDE characteristic frequency is defined as the temporal excitation frequency at which the currents within a primary winding of the winding structure transition from a nearly uniform distribution throughout the primary winding cross-section to a distribution in which the currents are confined to a thin layer near the surface of the primary winding. In many applications, the sensitivity of response measurements to specific MUT properties of interest, or the selectivity for two MUT properties of interest, is increased when the frequency of the electric signal is near the TDE characteristic frequency. As such, the TDE-based apparatus is intentionally constructed to amplify the effects of the TDE. To that end, the winding structure comprises an optional permeable substrate and an optional conducting backplane for tuning (i.e., intentionally altering) the TDE characteristic frequency of the winding structure.
The electromagnetic winding structure in the preferred embodiments comprises a plurality of electromagnetic windings forming a meandering pattern. The meandering winding structure is a significant feature of the invention in that its geometry provides physical behavior which may be accurately modeled. As such, the magnetometer is capable of accurately estimating preselected material properties based on sensed responses obtained by the meandering winding structure.
A TDE-based apparatus may further comprise a model which is successively implemented by the property estimator for generating a property estimation grid which translates sensed electromagnetic responses into estimates of preselected MUT properties. The model provides for each implementation a prediction of electromagnetic response for the preselected properties based on a set of properties characterizing the winding structure and the MUT. The model is described in more detail below in accordance with a method for generating a property estimation grid.
A TDE-based magnetometer may be manipulated to obtain multiple responses for various operating conditions. For example, sensed responses may be obtained by the winding structure at multiple proximities to the MUT and converted to material property estimates. In another example, sensed responses may be obtained by the winding structure for a plurality of positions along a surface of the MUT. Further, for each position relative to the MUT the winding structure may be adjusted to obtain sensed responses for various proximities and orientations relative to the MUT. In yet another example, the magnetometer winding structure may be capable of being adapted to conform to a curved surface of the MUT for obtaining sensed responses and providing estimates of MUT properties. In another example, the frequency of the electric signal may be varied for obtaining a plurality of frequency related sensed responses with the winding structure.
A TDE-based magnetometer may be employed in a plurality of specific applications to provide substantially independent estimates of specified properties of interest. To that end, a TDE-based magnetometer is capable of providing independent estimates of each of a pair of properties at a single temporal excitation frequency from a single sensed response. This enables the TDS-based magnetometer to obtain estimates of dispersive properties of single and multiple layered MUTs. Potential pairs of MUT properties include (1) conductivity and thickness, (2) conductivity and proximity, (3) conductivity and permeability, (4) thickness and permeability, (5) permeability and proximity and (7) the real and imaginary parts of the complex permeability. The MUT property estimates may then be processed to estimate other MUT properties such as aging/fatigue, bulk and surface crack location and heat affected zone properties.
In other preferred embodiments, it is desirable to shift the TDE characteristic frequency away from the characteristic transition frequency associated with magnetic diffusion in the MUT in order to measure preselected MUT properties with required levels of sensitivity, selectivity and dynamic range. This is accomplished by changing the physical and geometric properties of the magnetometer.
In one embodiment in which the TDE characteristic frequency may or may not be significant, a magnetometer has a winding structure comprising a primary winding capable of imposing a magnetic field in the MUT when driven by an electric signal. The winding structure also includes one or more secondary windings for sensing electromagnetic responses. In this winding structure, the primary winding has a width which is substantially greater than the width of the gap between the primary and secondary windings. Further, the width of the primary winding may also be substantially larger than the thickness of the primary. The winding structure of this embodiment preferably forms a meandering pattern. An analyzer is provided for applying an electric signal to the primary for imposing the magnetic field in the MUT, and a property estimator translates sensed responses into estimates of preselected MUT properties.
Preferably, the magnetometer employs a model which is implemented by the property estimator for generating a response prediction table which translates sensed electromagnetic responses into estimates of the preselected MUT properties.
As in the other embodiments, the magnetometer may be used to obtain multiple responses for a plurality of operating conditions. To that end, in one example the magnetometer winding structure may be capable of being adapted to conform to a curved surface of the MUT for obtaining sensed responses and providing estimates of MUT properties. In another example, the winding structure may be adjusted to obtain sensed responses at multiple proximities to the MUT. In yet another example, a magnetometer with a winding structure comprising a single primary and single secondary may be employed for estimating dispersive properties of an MUT with responses obtained at a single temporal excitation frequency and for single or multiple proximities to the MUT. In yet another example, the winding structure may be adjusted to obtain sensed responses for a plurality of positions along a surface of the MUT. Further, for each position the winding structure may be adjusted to obtain sensed responses for multiple proximities and orientations relative to the MUT. In another example, the frequency of the electric signal applied to the winding structure may be varied for each sensed response.
Devices which are constructed to incorporate both magnetoquasistatic (MQS) inductive coupling terms and electroquasistatic (EQS) capacitive coupling terms are referred to as M.S./EQS devices. These devices have applications for materials having properties of interest which are out of the dynamic range of existing M.S. magnetometers and EQS dielectrometers. The introduction of capacitive coupling corrections permits the extension of the dynamic range for MUT properties of interest by allowing responses to be obtained at temporal excitation frequencies at which capacitive coupling is significant in order to increase sensitivity to the MUT properties of interest.
Accordingly, in another embodiment of the invention, an M.S./EQS device provides estimates of distributed properties of a layered MUT. The M.S./EQS device includes an electromagnetic winding structure capable of imposing a magnetic field and an electric field in the MUT when driven by an electric signal and sensing electromagnetic and electric responses. The winding structure comprises a primary winding, a plurality of coplanar first secondary windings and an optional second secondary winding positioned in a different plane. An analyzer provides electric signals to the winding structure and a property estimator translates sensed responses into estimates of preselected properties of the MUT.
Depending on the application, the M.S./EQS device may be operated in an M.S. mode and/or an M.S./EQS mode and/or an EQS mode. Accordingly, when the input current temporal excitation frequency is within the M.S. range for the device, the input current is applied to the primary winding for imposing a magnetic field in the MUT. Sensed electromagnetic responses are obtained at the first secondary windings for each layer of the MUT. When the input current temporal excitation frequency is within the M.S. range and an input voltage temporal excitation frequency is within an EQS range, the input current is applied to the primary to impose a magnetic field and the input voltage is applied to the first secondary windings in a push-pull sense to impose an electric field in the MUT. Sensed electromagnetic responses are obtained at the second secondary winding for each layer of the MUT. When the input current temporal excitation frequency is within the EQS range, the input voltage is applied to the first secondary windings to impose an electric field in the material. Sensed electric responses are obtained at the primary winding for each layer of the MUT. The property analyzer is employed for translating the sensed responses into estimates of preselected distributed properties of each layer of the layered MUT.
The present invention also comprises a method for generating property estimates of one or more preselected properties of an MUT. Accordingly, an electromagnetic structure, an analyzer and a property estimator are provided. The first step in the method requires defining dynamic range and property estimate tolerance requirements for the preselected properties of the material. Next, a winding geometry and configuration is selected for the electromagnetic structure. A continuum model is used for generating property estimation grids for the preselected material properties as well as operating point response curves for preselected operating point parameters.
The grids and curves are subsequently analyzed to define a measurement strategy. Next, operating point parameters and a winding geometry and configuration are determined to meet the dynamic range and tolerance requirements. To accomplish this, property estimation grids and operating point response curves are generated and analyzed for various operating points. Next, sensed electro-magnetic responses are obtained at each operating point and converted by the property estimator into estimates of the preselected material properties. Property estimate tolerances are then estimated as a function of values of the estimated preselected properties over the defined dynamic range using the property estimation grids and operating point response curves. If the property estimate tolerance requirements are not achieved, the process is repeated for different operating point parameters and winding dimensions.
As stated previously, the property estimator implements a model for generating a property estimation grid which translates sensed responses into preselected material property estimates. Accordingly, the present invention includes a method for generating a property estimation grid for use with a magnetometer for estimating preselected properties of a MUT. The first step in generating a grid is defining physical and geometric properties for a MUT including the preselected properties of the MUT. Next, operating point parameters and a winding geometry and winding configuration for the magnetometer are defined.
The MUT properties, the operating point parameters and the magnetometer winding geometry and configuration are input into a model to compute an input/output terminal relation value. Preferably, the input/output terminal relation is a value of transimpedance magnitude and phase. The terminal relation value is then recorded and the process is repeated after incrementing the preselected properties of the MUT. After a number of iterations, the terminal relation values are plotted to form a property estimation grid.
The present invention also includes a method of selection of a magnetometer winding structure and operating point for measuring one or more preselected properties of an MUT which achieves specified property estimate requirements. The first step includes defining physical and geometric properties for the MUT including preselected properties of the MUT. Next, the magnetometer operating point parameters, winding geometry and winding configuration are defined.
The MUT properties and the magnometer operating point parameters, winding geometry and configuration are then input into a model for computing an input/output terminal relation value. The preselected properties of the material are then adjusted to compute another terminal relation value. Using the terminal relation values, Jacobian elements are computed. Note each Jacobian element is a measure of the variation in a terminal relation value due to the variation in a preselected material property.
Next, a singular value decomposition is applied to the Jacobian to obtain singular values, singular vectors and the condition number of said Jacobian. An evaluation is then made of the sensitivity, selectivity and dynamic range of the magnetometer winding structure and operating point parameters using the singular values, singular vectors and condition number. If the material property estimate requirements are not met, the process is repeated with adjusted magnetometer operating point parameters, and winding geometry and configuration until the material property estimate requirements are achieved.